For the record I dont really like how in Original Sin 2, poison does heal undead. Undead look like skeletons. So how would poison doing anything to them ?
I think D&D3 did this much better. Healing is infusion of positive life energy, Clerical Damage spells however are influsion of negative life energy, and indeed this heals the undead, while regular healing spells would hurt them. Mind Clerics would get both the heal and the harm spells this way, and they would be equally strong in how much they did. So that sort of made sense.
In D&D5 this is gone again though.
